Kiyothong, K., P. Rowlinson, M. Wanapat and S. Khampa A total mixed ration (TMR) containing a blend of feedstuffs naturally contaminated with harmful mycotoxins was fed for 84 days to 24 primiparous and multiparous Holstein–Friesian × local dairy cows in a randomised complete block design. The dietary treatments consisted of a contaminated TMR diet plus various levels of the mycotoxin deactivator product (MDP) (0, 15, 30 or 45 g/head.day). Deoxynivalenol (DON), fumonisin B1 (FB1), zearalenone (ZON) and ochratoxin A (OTA) were found in the TMR at levels up to 720, 701, 541 and 501 μg/kg, whereas aflatoxin B1 (AfB1) and T-2 toxin (T-2) were found in the TMR at levels of 38 and 270 μg/kg, respectively. Rumen microbial ecology, ruminal volatile fatty acid (VFA) concentrations, ruminal microorganism populations,feed intake, total tract digestibility, milk yield, milk composition and serum immunoglobulin (Ig) concentrations were measured. The results revealed that the ruminal pH, ruminal ammonia nitrogen (NH3-N) concentration, total ruminal VFA concentrations and ruminal bacterial counts were significantly (P < 0.05) higher in supplemented than in non-supplemented cows. Ruminal protozoal counts were significantly (P < 0.05) lower in supplemented than in non-supplemented cows. DM intake, and digestibility of crude protein (CP) and neutral detergent fibre (NDF) were significantly (P < 0.05) higher in supplemented than in non-supplemented cows. Serum IgA concentrations were significantly (P < 0.05) higher in supplemented than in non-supplemented cows. Milk yield and milk protein were significantly (P < 0.05) higher in supplemented than in non-supplemented cows. On the basis of this experiment, it can be concluded that milk production and feed intake can be increased with the addition of MDP to cow diet in the presence of mycotoxins. These increases were accompanied by decreases in the negative effects of mycotoxins on rumen and immune function